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Meiosis. Gamete production, takes place in reproductive organs/parts in both plants and animals Gametes have half the number of chromosomes (haploid), so when they come together the zygote (fertilized egg) will be diploid
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Meiosis • Gamete production, takes place in reproductive organs/parts in both plants and animals • Gametes have half the number of chromosomes (haploid), so when they come together the zygote (fertilized egg) will be diploid • If gametes weren’t haploid the offspring would have twice the correct number of chromosomes in every cell , for example: • Corn plant has 20 chromosomes (10 pairs) in every somatic cell, so its gametes have 10 single chromosomes • If it didn’t than the offspring would end up with 40 chromosomes (10 quads!) • 20 + 20 • 40 + 40 • 80 + 80 - 160 + 160 • 320 the 5th generation would have 32 of each of the 10 chromosomes!!
Meiosis, cont. • This is how the correct number of chromosomes is maintained from generation to generation • Called REDUCTION DIVISION— as cells divide, the number of chromosomes is reduced • Consists of 2 successive nuclear and cell divisions • Before Meiosis I, DNA replicates into fuzzy X’s • Meiosis I— homologous chromosomes pair up and then separate into 2 cells • Meiosis II—just like mitosis, but it happens to 2 cells simultaneously • Chromosomes line up at the center and sister chromatids are pulled apart • One “2n” (diploid) cell produces 4 “n” (haploid) cells
Interesting occurrences during meiosis….. • Crossing over—when homologous chromosomes exchange chromosome fragment • Causes genetic recombination and provides genetic variation (good thing!) • Happens during Prophase I, when chromosomes are coiling up and homologous chromosomes are pairing up • Shown happening at both ends of chromosomes, but can happen at just one • This is why you are not identical to your brothers/sisters
Another interesting occurrence…. • Nondisjunction—homologous chromosomes do not separate properly • Happens during Anaphase I or II • End result is two gametes end up with both chromatids (of that specific chromosome) and the other 2 cells end up without a copy of that chromosome • So if the gamete that has both of the chromatids is fertilized then that zygote will be trisomy (3 of one chromosome) for that chromosome • If the gamete that is missing that chromosome is fertilized, it will be monosomy (one chromosome) for that chromosome • Relatively common-- 1 in 5 normal human pregnancies miscarry w/in the first 2 months due to the baby having too many or too few chromosomes
